1. Field of the Invention
The present invention relates to a color noise slice method and a circuit for carrying out this method, and particularly relates to a color noise slice method and circuit for carrying out color noise slice processing to reduce noise in the chrominance signal component in digital image pickup output of image pickup elements.
2. Description of the Related Art
FIG. 1 shows the construction of an example of this type of color noise slice circuit of the prior art. Referring to FIG. 1, an output signal of image pickup element (IPE) 1, after prescribed analog processing such as sample hold or automatic gain control in analog processing circuit (AP) 8, is converted to a digital signal by analog digital converter (ADC) 2, and supplied to both luminance signal processing circuit (LSP) 3 and chrominance signal processing circuit (CSP) 4.
Luminance signal processing circuit 3 performs prescribed processing such as edge enhancement processing or gamma correction on the signal received from analog digital converter 2, and generates luminance signal Y. Delay circuit (DL) 31 performs delay processing to delay the luminance signal Y for a processing time necessary for color noise slice circuit 5 to process a chrominance signal. Description of color noise slice circuit 5 will be given in detail hereinbelow. The delay processing described above allows both luminance signal Y and color-difference signals (R-Y) and (B-Y) supplied from the same picture element simultaneously to be received by encoder 6 in the next stage.
Chrominance signal processing circuit 4 generates color-difference signals (R-Y) and (B-Y) after performing prescribed processing such as color demodulation, white balance processing, and gamma correction.
Color noise slice circuit 5 is provided with slice circuits 57 and 58 and noise slice width setting circuit 59. Noise slice width setting circuit (NSWS) 59 presets noise slice width AS. Slice circuits 57 and 58 receive color-difference signals (R-Y) and (B-Y) and also preset noise slice width .DELTA.S, perform the color noise slice processing described below of color-difference signals (R-Y) and (B-Y) using +.DELTA.S and -.DELTA.S as thresholds, and provide slice-processed color-difference signals (R-Y)' and (B-Y)', respectively:
When EQU (R-Y)&gt;+.DELTA.S, (R-Y)'=(R-Y)-.DELTA.S (1)
When EQU -.DELTA.S.ltoreq.(R-Y).ltoreq.+.DELTA.S, (R-Y)'=0 (2)
When EQU (R-Y)&lt;-.DELTA.S, (R-Y)'=(R-Y)+.DELTA.S (3)
and
When EQU (B-Y)&gt;+.DELTA.S, (B-Y)'=(B-Y)-.DELTA.S (4)
When EQU -(S.ltoreq.(B-Y).ltoreq.+.DELTA.S, (B-Y)'=0 (5)
When EQU (B-Y)&lt;-.DELTA.S, (B-Y)'=(R-Y)+.DELTA.S. (6)
Encoder 6 receives the Y signal, color-difference signal (R-Y)', and color-difference signal (B-Y)', and generates video output signals of a prescribed data format appropriate to video signals or appropriate to image processing such as NTSC, PAL, or SECAM.
FIG. 2 shows the input/output characteristics of a color noise slice circuit. FIG. 2 shows the case in which color-difference signal (R-Y)' and (B-Y)' are generated by slice-processing of the received color-difference signals (R-Y) and (B-Y) using .DELTA.S and -.DELTA.S as slice threshold values.
Generally, operation error or random noise generated by chrominance signal processing mixes with the color-difference signal and becomes color noise of a low signal level, which can be sliced by the above-described color noise slice processing to allow a signal having low color noise level and accordingly a good color signal-to-noise ratio.
FIG. 3 is a color vector diagram of a chrominance signal with color-difference signal (R-Y) as an ordinate and color-difference signal (B-Y) as an abscissa. In the figure, chrominance signals S1 and S6 denote the chrominance signals preceding and following the color noise slice processing, respectively. Here, the coordinate components of chrominance signals S1 and S6 are S1=(S1b, S1r) and S6=(S6b, S6r). In addition, the angles of chrominance signals S1 and S6 with respect to the (B-Y) axis are denoted by .theta..sub.1 and .theta..sub.6, respectively.
The relation between chrominance signals S6 and S1 in FIG. 3 is represented by: EQU S6b=S1b-.DELTA.S (7) EQU S6r=S1r-.DELTA.S (8)
As is known from the figure, the difference vector between vectors S1 and S6, i.e., (vector S1)-(vector S6), makes an angle of 45 degrees with respect to the (B-Y) axis and has a magnitude of 2.sup.1/2 * .DELTA.S. Accordingly, the vector S6, i.e. the chrominance signal after the slice processing, directs to the direction different from that of the vector S1, i.e., the chrominance signal before the slice processing.
FIG. 4 is a color vector diagram showing the range of chrominance signal S in which at least one of color-difference signal components (R-Y) and (B-Y) of chrominance signal S vanishes as a result of slice processing by color noise slice circuit 5. In this figure, such ranges of chrominance signal S is shaded by slanted lines. If the (R-Y) and (B-Y) components of chrominance signal S, i.e., color-difference signals R-Y and B-Y, are Sr and Sb, respectively, then the chrominance signal components Sr and Sb become 0 through slice processing when they are within the range represented by the following inequalities: EQU -.DELTA.S.ltoreq.Sr.ltoreq..DELTA.S (9) EQU -.DELTA.S.ltoreq.Sb.ltoreq..DELTA.S (10)
As explained hereinabove, operation error or random noise generated by chrominance signal processing frequently mixes with the color-difference signal and becomes color noise of a low signal level. For example, in the circuit shown in FIG. 1, even if a chrominance signal received by chrominance signal processing circuit 4 is only a blue signal, a color-difference signal R-Y of a low signal level is likely to be included as a color noise, in addition to the chrominance signal component B-Y, within the chrominance signal delivered from chrominance signal processing circuit 4. If this color noise R-Y is small compared to .DELTA.S, a chrominance signal containing this color noise will correspond to a color vector belonging to the slanted-line portion lying along the B-Y axis of the color vector space shown in FIG. 4.
The color noise component R-Y of this chrominance signal is eliminated after undergoing color noise slice processing by slice circuits 57, and the chrominance signal is transmitted as color-difference signal (B-Y)' (i.e., as a chrominance signal completely in the direction of the B-Y axis). Simultaneously, color-difference signal (B-Y) is also subjected to slice processing by slice circuit 58, and its signal level therefore lowers by .DELTA.S.
The same situation also holds true when the desired color-difference signal (color-difference signal not containing noise) is R-Y and the color noise is B-Y.
In the color noise slice processing circuit of the prior art described hereinabove, a problem has been that, since color-difference signal (R-Y) and color-difference signal (B-Y) are both subjected to slice processing using the same .DELTA.S or -.DELTA.S as the threshold value, signal S1 preceding slice processing and signal S6 following slice processing inevitably have differing angles .theta..sub.1 and .theta..sub.6 with respect to the (B-Y) axis, as shown in FIG. 3.
In other words, received chrominance signal S1 is altered by noise slice processing to a chrominance signal having different phase in color vector space, i.e., it changes to a chrominance signal of a different color. Hereinbelow, the phase in the color vector space will be referred to as "hue".
In addition, because a desired chrominance signal of a low signal level vanishes as a result of the slice processing, faithful image reproduction is degraded. Hereinbelow, a signal level will be referred to as a saturation.
Moreover, a chrominance signal of high saturation normally includes a color noise component of low saturation .delta.S which accompanies a desired chrominance signal of high saturation S.sub.0 and varies with time. In such a case, because color noise slice processing reduces only the desired chrominance signal S.sub.0, and because the varying noise component .delta.S accompanying the desired chrominance signal S.sub.0 does not change, the color signal-to-noise ratio is degraded by color noise slice processing.
As an example, let a desired color-difference signal B-Y of high saturation be accompanied by a time-dependent color noise .delta.Sb(t), while let a desired color-difference signal R-Y of high saturation be accompanied by time-dependent color noise .delta.Sr(t). Then, if .delta.Sb and .delta.Sr change in a manner that satisfies the following equation, EQU .delta.Sb/.delta.Sr=Sb/Sr, (11)
then the hue of chrominance signal S+.delta.S will not vary with time even when the time-dependent noise .delta.S superposes on the desired chrominance signal. In this case, although the time-variation .delta.S(t) of chrominance signal S causes saturation noise, it does not result in color noise (hue noise).
However, the time-variation in saturation of the color-difference signal generated through chrominance signal processing does not generally satisfy equation (11). As a result, fluctuation in a color-difference signal results in simultaneous saturation noise and color noise.
When a color-difference signal of time-varying and high saturation (R-Y).sub.0 +.delta.Sr(t) is supplied to the color noise slice circuit 57, the slice circuit 57 carries out following processing in accordance with equation (1); EQU (R-Y)'=(R-Y).sub.0 +.delta.Sr(t)-.DELTA.S=(R-Y).sub.0 -.DELTA.S!+.delta.Sr(t) (12)
As is clear from equation (12), time-variation .delta.Sr(t) is not eliminated by subtraction of constant .DELTA.S from the varying chrominance signal and remains unchanged in the slice-processed color difference signal (R-Y)'. Moreover, because the desired chrominance signal (R-Y).sub.0 as well undergoes slice processing, the signal-to-noise ratio becomes .vertline.(R-Y).sub.0 -.DELTA.S!/.delta.Sr.vertline., which is smaller than the signal-to-noise ratio .vertline.(R-Y).sub.0 /.delta.S.vertline. preceding color noise slice processing. In addition, digital color noise slice processing entails the problem of increased circuit scale.
The present invention has been developed in view of the above-described problems of the prior art and is directed toward providing a color noise slice processing circuit and color noise slice processing method that do not bring about changes in hue in color noise slice processing; that do not vanish the chrominance signal of a prescribed hue through color noise slice processing even if the signal level is low; that do not cause a decrease in the signal-to-noise ratio in noise slice processing of a chrominance signal having a prescribed hue; and finally, that allow a reduction in circuit scale.